Slavery is the act of one sentientbeing controlling another in violation of the other's will. The one so controlled is referred to as a slave. Slaves are often considered the property of the controller.

The Orions used slave labor as late as the 23rd century. Common belief was that Orion women were slaves to the men, when it was in fact the opposite. In the 22nd century, Orions frequently kidnapped members of other species for slave trade. (ENT: "Borderland", "Bound")

The Kazon were a subjugated race, used as slave labor by the Trabe who had conquered their homeworld. They remained in captivity until in 2346, when JalSankur convinced the various Kazon sects to put aside their differences and rise up against the Trabe. In doing so the Kazon took the Trabe's ships and technology and forcing them to become a nomadic species, never allowing them to settle on a new world. (VOY: "Initiations", "Alliances")

When Bruce Maddox wanted to disassemble Data in 2365 in order to learn how to create more androids, Data refused, but was ordered to undergo the procedure on the grounds that he was Starfleet property. Jean-Luc Picard challenged the ruling but questioned his convictions after hearing the statement of the prosecution. Guinan helped him realize that creating an entire "race" of Datas for use as tools would, in fact, be slavery. (TNG: "The Measure Of A Man")